Modern solar cells typically include a large-area, single layer p-n junction diode that is capable of generating electrical energy from solar light. These cells are typically made using silicon wafers that are doped to include one or more n-type doped regions, and one or more p-type doped regions. Such solar cells (also known as silicon wafer-based solar cells) are currently the dominant technology in the commercial production of solar cells, and are the main focus of the present invention.
A desirable solar cell geometry, commonly referred to as the interdigitated back contact (IBC) cell, consists of a semiconductor wafer, such as silicon, and alternating lines (interdigitated stripes) of p-type and n-type doping. This cell architecture has the advantage that all of the electrical contacts to the p and n regions can be made to one side of the wafer. When the wafers are connected together into a module, the wiring is all done from one side. Device structure and fabrication means for this device have been described previously in co-owned and co-pending U.S. patent application Ser. No. 11/336,714 entitled “Solar Cell Production Using Non-Contact Patterning and Direct-Write Metallization”, which is incorporated herein by reference in its entirety.
One method for forming the alternately doped line regions in an IBC solar cell is to dispose dopant bearing pastes of alternating dopant type on the wafer, and then to heat the wafer with the appropriate temperature profile to drive in the dopants. Solar cell doping and the patterning of doped regions is typically carried out with costly steps that may include the use of barrier deposition, barrier patterning, laser processing, damage removal, and gas phase furnace diffusion. One could also generate the desired interdigitated doped regions using screen printing techniques. However, a distinct disadvantage of screen printing is that two separate print operations would be needed to write the two dopant bearing materials, and the two prints would need to be exquisitely well registered. Moreover, screen printing requires contact with the wafer, which increases the risk of wafer damage (breakage), thus increasing overall production costs. In addition, the first screen printed layer needs to be dried before a second screen print step is applied.
One commonly used solar cell architecture utilizes the back surface of the cell wafer as a broad area metal pad, typically aluminum, to form a contact to the p-type side of the device. During the metal firing step, the aluminum interacts with the silicon to form a p+ doped layer. In some cases, the back surface is also doped with boron to produce a p+ layer. The role of this layer is to create a so-called back surface field which reduces the recombination of the photocurrent on the back metallization. The broad area metal layer is commonly applied either by screen printing or pad printing, both of which are contact printing methods, and therefore increase the risk of wafer breakage.
What is needed is a low cost method and system for producing doped regions in solar cell substrates that avoids the problems associated with contact printing methods. In particular, what is needed is a simpler and more reliable method for producing self-registered p-type and n-type doped regions in the production of IBC solar cells.